Recently, a semiconductor pressure sensor is proposed, for example, in JP-2871064 (JP Application Laid-open No. H04-162779). The semiconductor pressure sensor 110 is constructed as shown in FIGS. 8A and 8B, for detecting hydraulic pressure. It includes a housing 16, a seating 1a made of Pyrex glass, a N-type silicon (i.e., Si) substrate 3a bonded on the seating 1a, four piezoresistors 5a formed on one surface of the substrate 3a, an insulation film 6a formed on the substrate 3a, signal electrodes 7a, and a fixed potential electrode 8a. 
Each piezoresistor 5a is formed by doping boron particles in the substrate 3a. The substrate 3a includes a diaphragm part 80a, which is disposed on the seating la surface that is opposite to the piezoresistor 5a, so that the diaphragm part 80a contacts water as an object of measurement. The water is introduced from an outside of the sensor 110 through a passage (not shown). Each signal electrode 7a, the fixed potential electrode 7a, and an electrode pin (not shown) are connected with an aluminum wiring layer (not shown). The aluminum wiring layer is formed on the insulation film 6a. 
Four piezoresistors 5a form a bridge circuit, as shown in FIG. 8B. A bias voltage Vb is applied between the fixed potential electrode 8a and a maximum potential point VL that has the highest potential in the bridge circuit. Because of the bias voltage Vb, a reversed bias is applied to a PN junction in the sensor 110, so that a depletion layer of the PN junction is formed. This depletion layer has a capacitance 12 so that the capacitance 12 reduces an alternating current noise mixing into the sensor 110 by passing through the capacitor of the depletion layer.
However, a semiconductor pressure sensor is required to detect not only hydraulic pressure but also oil pressure, atmospheric pressure and the like. Especially, a semiconductor pressure sensor mounted on an automotive vehicle is required to detect pressures of several fluids. Therefore, each sensor has a different construction, and is mounted on a different position of the vehicle. Thus, the semiconductor pressure sensor is required to be high noise protection.